In hot strip mills, hot, relatively thick slabs or strips of steel are rolled into thin strips by passage between work rolls of one or more rolling mill stands. During a metal rolling operation, mill rolls are continuously heated by a work heat due to the plastic deformation of the rolled metal, a frictional heat generated between the rolled metal and the rolls, and, in case of hot rolling, heat transfer from a hot metal workpiece. Particularly in the case of hot rolling steel where the steel to be rolled is preheated to temperatures in excess of 1200.degree. C., roll heating as a result of heat transfer can become rather excessive.
Because of such roll heating, it is imperative that means be provided to cool the rolls during use and thereby prevent unwanted thermal expansion of the rolls, which can adversely affect the quality of the rolled product. For example, some rolls tend to become excessively heated in their mid-portion, causing the diameter to increase only in the mid-portion, and therefore roll a thinned mid-section into the product as compared to the outer sections. In addition, excessively heated rolls will wear more quickly and tend to stick to the rolled metal surface to adversely affect the surface quality of the rolled product.
Also, during the hot strip mill rolling operation the strip of metal being rolled must be cooled so as to achieve a predetermined microstructure and thus the desired physical properties in the hot rolled product before the slab or strip of metal reaches the rolling stand. Generally, for steel, the cooling rate should be fast enough to achieve the transformation to a desired ferrite microstructure but not so fast as to quench the steel to a condition resulting in brittleness.
Numerous types of cooling apparatus have been proposed, with normally one device used for cooling of the work rolls, while a second device is used for cooling of the hot metal strip.
A conventional cooling system for work rolls and a metal strip being rolled is illustrated in FIG. 1. As illustrated, work rolls 1 and 2 are provided through which a hot strip of metal 3 is passed, the work rolls 1 and 2 provided with respective back-up rolls 4 and 5. The hot strip of metal 3 is moved to and through the work rolls 1 and 2 by the use of table rolls 6, 7 and 8, with aprons 9 and 10 also provided. The lower work roll 2 is cooled by application of a coolant fluid 11, such as water, onto the work roll 2 through a lower coolant fluid supply header 12. An upper coolant fluid supply header 13 is also provided to direct a supply of a coolant fluid 14 onto the upper work roll 1. A wiper or stripper 15 is positioned between the upper coolant fluid supply header 13 and the strip of metal 3, which has a wiper blade 16 that contacts the surface 17 of upper work roll 1 and directs excess coolant fluid 14 off to the sides of the strip of metal 3 and prevents contact of the fluid coolant 14 with the upper surface 18 of the strip of metal 3. The stripper 15 is arranged to contact the upper work roll 1 by support on a stripper guide 19, with a stripper retracting fluid cylinder 20 provided which, through rod 21 and connection 22, is adapted to provide a wiping contact of the wiper blade 16 of stripper 15 against surface 17 of the upper work roll 1, the fluid cylinder being supported by a support frame 23. In order to cool the strip of metal 3, a strip cooling header 24 is disposed above and across the strip of metal and a coolant fluid 25 is directed from the strip coolant header 24 onto the upper surface 18 of the hot strip of metal 3.
While such a roll and strip coolant system is conventionally used, there are disadvantages associated with such a system. One disadvantage lies in the need for two coolant fluid manifolds and delivery systems, one for the work roll 1 and another for the strip of metal 3. Another disadvantage exists in that the coolant fluid contact with the strip of metal is made at location distant from the bite of the work rolls 1 and 2, such that there is enough time for the surface 18 of the strip of metal 3 to rebound in temperature because of the heat transfer from the inner portions of the strip of metal. Such rebound in temperature of the surface 18 of the strip of metal 3 results in increased rescaling of the strip surface 18 and subsequently increased scale pickup by the work roll 1.